couldn`t be, cuz i`m sure lots of empires would pay 5 btc (the stephen handout) for a month. It would have to be 30 btc and some regalos and/or gordos thrown in.
BTCs dont have great value aless you get them bang on when you want them,
however on slow servers, few extra turns have great value even when u dont need em because you can speed up reseach, attack more targets blah blah blah
i would say for an RT paid, i would part for like 600turns/1000 turns would make me happy :p
in terms of gordos however, depending on who you talk to, you could sell a paid for somewhere between 25-30 gordos
other artifacts arent really common paid currencys, i spose regs are used on occasion, but any other artifacts youd simply have to work somethin out with the seller/buyer